Laser-Doppler velocimetry is the technique whereby the velocity of a particle traversing a measurement region is measured by illuminating the measurement region with coherent light, collecting light scattered by the particle, and using interference effects to manifest the frequency shift in the scattered light due to the velocity of the particle.
Laser-Doppler anemometry is the application of laser-Doppler velocimetry to measurement of velocity of flow of a gas, and is described in F. Durst et al, Principles and Practice of Laser-Doppler Anemometry, Academic Press 1976. One of the methods described by Durst et al for carrying out a laser-Doppler anemometric measurement is referred to as the dual beam method. The dual beam concept may be implemented in a procedure that may be described as the off-axis, dual beam backscatter method. Two parallel beams of light from a laser are converged by a lens and caused to intersect in a measurement volume that lies in the flow of a gas that has small particles entrained therein. Light that is backscattered from the particles in the measurement volume is collected by a second lens that is off-axis with respect to the first-mentioned lens, and backscattered light waves originating from the two beams respectively interfere. If the solid angle from which the backscattered waves are collected is sufficiently small, a photodetector can be used to provide an output signal wherein the frequency is dependent on the velocity component of the particles along an axis that lies in the plane that contains the converging beams and is perpendicular to the bisector of the angle between the two converging beams.
Laser-Doppler anemometry has several advantages when applied to measurement of the velocity field in a wind tunnel. In particular, it is not necessary to introduce a physical measuring probe into the test section of the wind tunnel, and therefore the only effect of the measurement on the flow is due to the particles themselves. By appropriate selection of the particle size and density, the effect of the measurement technique on the flow is rendered substantially negligible. Moreover, since it is not necessary to introduce a physical probe into the test section, the position of the measurement volume can be changed without its being necessary to move any physical elements that are within the test section.
It is desirable that the lens used in the off-axis, dual beam backscatter method for converging the parallel beams on the measurement volume be close to the measurement volume, and therefore efforts to apply this method to measuring flow velocity in the test section of a trans-sonic wind tunnel have involved mounting the laser and associated optics in the plenum of the wind tunnel. The plenum of a trans-sonic wind tunnel is a hostile environment with respect to proper operation of a laser. For example, the pressure in the plenum may be as low as one-half of normal atmospheric pressure at sea level, and therefore arcing between the electrodes of the laser is liable to occur unless special steps are taken to prevent it. Moreover, at flow speeds of about Mach 1, severe vibrations occur, and the temperature in the plenum can be quite high (about 60.degree. C). It has therefore been proposed that the lasers and associated optics be contained in a sealed, temperature-contorlled casing mounted in the plenum. However, this solution to the problem posed by the hostile environment in the plenum of a trans-sonic wind tunnel is subject to disadvantage since the casing is rather bulky and cannot readily be accommodated in the plenum.
It is an object of the present invention to provide an improved apparatus for accurately measuring wind tunnel velocity without requiring delicate instruments or the like to be placed in a hostile environment.